The major things that seem to be Nick's advantages are his sense of right and wrong and a sense of decency that helps him to stand apart from some of the madness that he will encounter as the story progresses.

His father urged him not to judge others since not everyone had been given such a clear moral education or expected to have a clear set of values.

As the narrator of the story, in some ways he certainly can be judged, but the distance he maintains from most of the characters in a certain way does make it more difficult to judge him the same way a reader might the other characters.